1. Field of the Invention
The present invention relates to a zoom lens system that is provided for imaging.
2. Description of Related Art
In the field of manufactured products such as photographic cameras, electronic still cameras and video cameras, it has become standard practice in recent years to use a zoom lens system. Furthermore, even in the past it has always been common for the market to demand a higher optical quality, a higher zoom-ratio, and compactness.
Types of zoom lens systems having various refractive-power distributions depending on the magnification and use, are known in the art. As examples of a compact and high-quality type of zoom lens system, a zoom lens system arrangement having three lens groups, i.e., a negative first lens group, a positive second lens group and a positive third lens group, in that order from the object side, is disclosed in Japanese Unexamined Patent Publication Nos. 2004-239973, 2008-203881, 2008-203449 and 2008-241794.
However, such a zoom lens system that has three lens groups, i.e., a negative lens group, a positive lens group and a positive lens group, in that order from the object side, typically only has a small zoom ratio and is not very suitable for obtaining a higher zoom ratio while maintaining compactness. In order to achieve a high zoom ratio and compactness with such an arrangement, it becomes imperative to reduce the amount of movement of each lens group by strengthening the refractive power of each lens group. The refractive-power distribution in regard to the second lens group, which carries the major burden for zooming, is particularly important. Hence, increasing the refractive power of the second lens group, so that the amount of movement of the second lens group that occurs during zoom decreases, contributes greatly to achieving a compact zoom lens system. However, if the refractive power of each lens group is strengthened, aberration fluctuations also increase in each lens group during zooming, so that it typically becomes difficult to achieve a zoom lens system having a high optical quality throughout the entire zooming range.
For example, in the aforementioned Japanese Unexamined Patent Publication Nos. 2004-239973 and 2008-203881, since the refractive power and magnification setting of the second lens group is unsuitable, the amount of movement of the second lens group from the short focal length extremity (wide-angle extremity) to the long focal length extremity (telephoto extremity) is large, which causes an increase in the overall length of the zoom lens system. Furthermore, although the zoom lens system disclosed in aforementioned Japanese Unexamined Patent Publication No. 2008-203449 is extremely compact, since the refractive power and magnification setting of the second lens group is likewise unsuitable, the zoom ratio thereof is less than 3:1. Moreover, the zoom lens system disclosed in aforementioned Japanese Unexamined Patent Publication No. 2008-241794 achieves a high zoom ratio of 6:1, however, the overall length of the zoom lens system from the short focal length extremity to the long focal length extremity is enlarged. In this example also, the insufficient magnification of the second lens group, especially the magnification at the long focal length extremity, causes an enlarged overall length of the zoom lens system.
Furthermore, on the other hand, the demand for telecentricity in digital cameras of recent years can be cited as a reason for hindering compactness. Ideal telecentricity of a lens system is defined as when the principal rays at every angle-of-view are incident on the image sensor at a perpendicular angle thereto, and therefore, since such an arrangement takes away freedom of design, this often conflicts with achieving a compact lens system. In the zoom lens systems disclosed in aforementioned Japanese Unexamined Patent Publication Nos. 2004-239973 and 2008-203881, since the rearmost lens element (closest to the image side) within the second lens group has a positive refractive power, the height of the light bundle that is incident onto the third lens group is low, so that it becomes difficult to obtain telecentricity. Hence, it is difficult to achieve a zoom lens system which has three lens groups that exhibit both a high zoom-ratio and compactness.
Furthermore, in general, when designing a zoom lens system having a multiple number of lens groups, if the refractive power of each lens group is strengthened and the number of lens elements is decreased due to the pursuit of compactness and of a high zoom-ratio (high performance), the remaining amount of aberration at each lens group increases, or the amount of aberration fluctuations that occur during zooming increases, so that it becomes extremely difficult to achieve a sufficient optical quality together with a sufficient compactness and a sufficiently high zoom-ratio.